Access points (APs) are capable of communicating with a large number of associated stations (STAs). The STAs may be, for example, battery powered sensors which transmit and receive data on rare instances and which remain in a low power operation mode for relatively long periods of time. The Institute of Electrical and Electronics Engineers (IEEE) 802.11ah standard defines the communication between stations and access points in the sub 1 gigahertz (GHz) band. The 802.11ah standard describes several different use cases including the use of stations to serve as sensors and meters, to provide backhaul sensor and meter data and to provide extended range wi-fi.
With regard to the use of stations as sensors and meters, the access point may be capable of supporting a large number of associated stations, such as over 6,000 stations, which may operate on a very strict energy budget. The stations may be either near to or far away from the access point, such as by being located anywhere within a one kilometer radius from the access point. The stations may be battery powered sensors which are configured to transmit and receive data on rare occasions and to remain in a low power mode of operation for relatively long periods of time between the transmission and reception events.
With regard to the use of stations to backhaul sensor and meter data, the 802.11ah standard provides a backhaul link to support the traffic from the sensors embodied by the stations and to stream images or other traffic. In this case, the access point can aggregate multiple remote input/output points. With regard to the use of stations to extend the range of wi-fi, an access point operating in accordance with the 802.11ah standard may provide for an extended range hotspot, such as for use at home, on a campus or within a shopping mall, and may serve to offload traffic from a cellular or other network.
The above three example use cases each have different characteristics and requirements. For example, the three use cases have different requirements in terms of traffic load ranging from relatively low data rates in instances in which the stations serve as sensors or meters to relatively high data rates in instances in which the stations serve to extend the range of wi-fi and to offload traffic from a cellular or other network. The three use cases also have different characteristics in terms of traffic type, such as by supporting traffic having a bursty characteristic in an instance in which the station serves to extend the range of wi-fi by offloading traffic from a cellular or other network in comparison to supporting traffic having a relatively low duty cycle in an instance in which a station serves as a sensor or meter that may only provide data on an hourly or daily basis. The three use cases may also have different sleeping requirements, may place the stations in different environments, such as rural environments, urban environments, suburban environments, home environments, etc., and may support different data rates ranging, for example, from 10 Mbps for a station that serves to extend the range of an outdoor hotspot to 100 kbps for a station that serves as a sensor or a meter.
An access point may have multiple modes of operation, such as a sensor only mode in which the access point only supports stations configured as sensors and meters, an offload only mode in which the access point only supports stations configured to offload traffic from a cellular or other network only or a mixed mode in which the access point supports a variety of different types of stations including stations that serve as sensors and meters and stations that serve to offload traffic from a cellular or other network. The access point may advertise its mode in the beacon and probe response frames. Once the access point has advertised its mode of operation, the station may associate with an access point that serves a particular type of station. For example, a station that serves as a sensor may wish to associate with an access point that is operating in a sensor only mode or a mixed mode, but may not wish to associate with an access point that is operating in an offload only mode.
In order to support communication between the access point and the stations, a period of time termed a restricted access window (RAW) may be defined. The RAW is divided into a plurality of time slots and only certain stations may access the wireless medium during the respective slots. The access point may assign stations to the different time slots using a function. This function may depend on the association identification (AID) of the station, among other parameters. Further details are provided by IEEE 802.11-12/0831r0 and IEEE 802.11-12/1321r0. In this regard, a station-slot mapping function f(x) as set forth by IEEE 802.11-12/1321r0 may be employed to allocate stations to slots of the RAW. The station-slot mapping function f(x) may be defined to equal (x+Noffset) mod NRAW=i with the result i being the slot index assigned to a respective station. The slot index identifies the slot within the RAW to which the respective station is assigned. In an instance in which the RAW is for both paged and unpaged stations, x may be the AID of a station. However, in an instance in which the RAW is restricted to only the paged stations, x may be the position index of a paged station among all paged stations when sequentially arranged based upon their AIDs. For example, in an instance in which the stations are sequentially arranged based upon their AIDs and there are n paged stations prior to the paged station in question, x may equal n assuming the first paged station's bit position is defined to be x=0. Additionally, Noffset is an offset value in the mapping function that is provided to address fairness among the stations indicated in the traffic indication map (TIM). The offset may be provided by an existing field of the beacon, such as the time stamp, frame check sequency (FSC) or the like. Additionally, mod X indicates a modulo X operation.
In an instance in which the access point operates in a sensor only mode or a offload only mode, the AID assignment function may correctly allocate the stations to the RAW slots since each station assigned to a respective slot will be the same type of station, such as a station serving as a sensor or a station supporting offloading. However, in an instance in which the access point operates in a mixed mode, the access point and its AID assignment function cannot distinguish between the AIDs of different types of stations, such as stations that serve as sensors and stations that support offloading. As such, different types of stations may be assigned to the same slot of the RAW. The assignment of different types of stations to the same slot may create issues as the different types of stations may have different traffic requirements and characteristics. For example, in an instance in which a station that serves as a sensor is allocated to the same slot in the RAW as stations that serve to offload traffic, the station that serves as a sensor may never have an opportunity to transmit its sensor data since the duration of the slot may be fully consumed by the station that serves to offload traffic.
An access point that operates in a mixed mode may initially endeavor to allocate the same type of stations to the same slot within the RAW by, e.g., trying to assign AIDs to the slots in an intelligent fashion so that the AID assignment function would allocate the same type of stations in the same slot and prohibit different types of stations from accessing the same slot. However, this assignment process can be complicated, especially since the AIDs can be assigned based on the sleeping requirements of the stations. The existence of two constraints, based on the sleeping requirements and on the station type, that need to be combined for the AID assignment makes the process employed by the access point complicated. Additionally, the assignment of stations of the same type to the slots of the RAW may also be challenging in an instance in which the number of slots in the RAW changes since the station-slot mapping function f(x) would also change. Every time that the number of slots changes the access point would need to reassign the AIDs to the stations to guarantee that the same type of stations are allocated to the same slot. This reallocation process may be prohibitively resource intensive since the access point would need to communicate with each of the stations, which may number in the thousands in some instances.
The beacon transmitted by an access point may include a set of parameters for the RAW access. This set of parameters is called the RAW parameter set. Further details are provided by IEEE 802.11-12/843r0. Although the RAW parameter set defines various aspects of the RAW, the stations that may access the current RAW is determined by the current page segment of the traffic indication map (TIM). Furthermore, the mapping function of the access point that assigns the stations to slots in the RAW may take into account if the stations are paged in the TIM or not, but is completely independent of the type of station. Thus, different types of stations may be assigned to the same slot in an instance in which an access point operates in a mixed mode which may, in turn, create difficulties for at least some of the stations to communicate with the access point.